The invention relates to a process for the production of aqueous two-component polyurethane lacquer emulsions, with variable throughput, and to the use thereof as high-quality lacquers and coatings.
Two-component polyurethane lacquers (2C-PUR lacquers) are not mixed until shortly before application, because of the limited processing time (working life) of the lacquers. Depending on the reactivity of the lacquer systems, the said working life may be from several minutes to several hours.
While such two-component systems have in the past been used in solution in organic solvents, a number of water-dispersible two-component systems have recently been developed. Water-dispersible two-component systems normally consist of a resin component containing hydroxyl groups (binder, polyol), and a polyisocyanate component (hardener, crosslinking agent). The hydroxy-functional resin component is generally present in the form of an aqueous dispersion, and the polyisocyanate component is generally in the form of an anhydrous one-hundred-percent component or is in solution in a solvent. Such systems, which can also be used within the scope of the present invention, are disclosed, for example, in EP-A 358 979, 496 205, 469 389, 520 266, 540 985, 542 105, 543 228, 548 669, 562 282 and 583 728. A disadvantage of those lacquer systems is that the known lacquer quality of two-component systems based on purely organic solvents is not achieved in some fields of application. That is true especially of fields of application in which particularly high optical properties and resistance properties are required.
In order to achieve lacquer surfaces of high quality, it is known to use lacquer dispersions the particle size of which is as small as possible. In aqueous two-component polyurethane lacquers, therefore, polyol dispersions having a sufficiently small particle size of less than 500 nm, preferably from 10 to 200 nm, are generally used. Dispersion of the isocyanate component, which is hydrophobic per se, does not take place until shortly before application of the lacquers, since the polyisocyanate component reacts with water and therefore has only a limited storage stability in the presence of water. Dispersion of the polyisocyanate component, which is hydrophobic per se, in the aqueous hydroxy-functional resin dispersion by means of conventional static mixing devices causes considerable difficulties, however. The reason is that the isocyanate component is stabilised during the emulsification at the surface of emulsion particles that have already formed, so that the superficial stabilising layer stands in the way of further division. Aqueous two-component polyurethane lacquer emulsions therefore normally have a bimodal particle size distribution, with a first distribution maximum at a particle size corresponding to the hydroxy-functional resin dispersion and a second distribution maximum at a particle size greater than 10,000 nm (isocyanate component), considerable portions having particle sizes greater than 20,000 nm still being present.
Polyisocyanates that have already been rendered hydrophilic by chemical modification and polyisocyanates containing external emulsifiers have also been developed; although such polyisocyanates permit markedly simpler dispersion to an average particle size of less than 1000 nm using static mixing devices, they yield hardened lacquer films that have resistance properties which are inadequate for many fields of application. Lacquer films having good resistance properties, however, are obtained only by the use of hydrophobic polyisocyanate components.
On the basis of the conception that the dispersibility of the isocyanate component is limited by the stabilisation reaction which takes place at the surface of particles that are already present, possible ways have been sought of achieving as finely divided a distribution as possible within periods of time which are so short that noticeable superficial stabilisation does not yet occur. In particular, warming which accelerates the reaction of the polyisocyanate component with water is also to be avoided during the dispersion.
In DE-A 19 510 651 it is described that, after pre-mixing of the hydroxy-functional binder dispersion with the polyisocyanate component, water, optionally additional solvent and, optionally, other lacquer auxiliaries known per se, the resulting pre-emulsion, of which no particular requirements are to be made, is forced at high pressure through a nozzle having small measurements in at least one dimension. There are mentioned as homogenising nozzles slit-type nozzles, circular-slot nozzles or hole-type nozzles. Special preference is given to the use of a jet disperser according to EP-A-101 007, since very finely divided dispersions are formed even at relatively low pressures. The pressure to be applied is to be from 1 to 30 MPa (from 10 to 300 atmospheres), in the case of the jet disperser preferably from 1 to 8 MPa, especially from 2 to 6 MPa. By means of that emulsification it is possible to produce two-component polyurethane lacquer emulsions which are stable for several hours and which exhibit a considerably improved surface quality after application and hardening.
It is also possible to reduce the content of solvent in the dispersion considerably and preferably to dispense with rendering the polyisocyanate component hydrophilic. In particular, dispersions having a solvent content of less than 15% can readily be produced in accordance with the invention. Depending on the pressure applied during the dispersion, the number of nozzle passages and the two-component system used, it is also possible to produce emulsions that are completely free of solvents and agents imparting hydrophilicity.
The high lacquer surface qualities achievable with the process according to the invention can be related directly to the particle size distribution of the emulsions.
In DE-A 19 510 651, the use of the jet disperser having fixed dimensions is described in FIGS. 1, 2, 3, 4 and 5. FIG. 6 describes a particular form in which the bores are freed or closed off by a movable application pipe. This form has proved to be impractical for use. It has been found that such homogenising nozzles must be adjustable very rapidly in fractions of a second in order to produce the same emulsion quality continuously when the amounts deposited vary.
It has been found that motor vehicle bodies, for example, can be lacquered especially advantageously at a very high quality if the polyisocyanate is continuously emulsified in the aqueous polyol component immediately before being introduced into the spray gun or atomiser bell. Problems arise, however, if the amount of lacquer taken varies within very short intervals of time owing to the geometry of the motor vehicle body.
The object of the invention was, therefore, to make available a mixing apparatus for aqueous two-component polyurethane lacquers having a high lacquer quality, which mixing apparatus continuously produces the same emulsion qualities when the amounts deposited vary.
Surprisingly, it has been possible to achieve that object by means of the embodiment of a jet disperser described in greater detail below.